A conventional wireless battery charging system 10 for charging a rechargeable traction battery 20 for supplying electrical energy to an electric traction motor 16 of an electric vehicle 14 is illustrated schematically in FIG. 1. The system 10 comprises a stationary charging system device 220 and a vehicle-side, electronic charging system device 26. The stationary charging system device 220 serves to transmit energy via a wireless, for example induction-based, link 12 to the vehicle-side charging system device 26 and via said vehicle-side charging system device into the traction battery 20 of the electric vehicle 14. The vehicle-side charging system device 26 in this case serves to receive, convert and feed energy into the traction battery 20.
The vehicle-side charging system device 26 comprises a first LC resonant circuit 28, which is designed to receive energy from the charging device 220, and a rectifier device 86 comprising a current rectifier 96 (see FIG. 2), which is designed to convert an AC electric voltage applied to its AC voltage inputs 98 and 100 (see FIG. 2) into a DC electric voltage provided at its DC voltage outputs 102 and 104 for charging the traction battery 20.
The stationary charging system device 220 comprises a grid supply connection 244, a control device 242 and one or more second LC resonant circuits 222. The stationary charging system device 220 is connected to the public electricity grid via the grid supply connection 244 and can draw electrical energy. Via the control device 242 or controlled thereby, the electrical energy is supplied as AC energy to one of the second LC resonant circuits 222, which is designed to convert the electrical energy into electromagnetic energy and to emit said energy so that some of the electromagnetic energy emitted is received via the wireless link 12 from the first LC resonant circuit 28 acting as receiver, is converted into electrical AC voltage energy and as such is supplied to the current rectifier 96, which converts the energy into DC energy for charging the traction battery 20. The DC voltage energy is fed from the vehicle-side charging system device 26 via the rectifier device 86 into the traction battery 20.
In order that a traction battery 20 can be charged wirelessly via a stationary charging device 220, the electric vehicle 14 finds a parking space where it is parked for the duration of the charging process so that the wireless link 12 can be set up from one of the second LC resonant circuits 222 of the stationary charging system device 220 to the first LC resonant circuit 28 of the vehicle-side charging system device 26. The electric vehicle 14 logs on via a likewise wireless radio link in the stationary charging system device 220 and exchanges various information with respect to the charging process with said stationary charging system device, including the state of charge of its traction battery 20, charging times, available power, power requirement, electric voltages, energy quantity and prices. Furthermore, safety-relevant data, including overvoltages, overheating and other possible system fault states and diagnosis data, are exchanged in both communications directions.
The two units of the battery charging system 10 which are connected wirelessly to one another, namely the stationary charging system device 220 and the electric vehicle 14, can assume states over the course of the charging process which need to be communicated to the respective other unit in order that the other unit can respond correspondingly. Examples of these states are reaching of the end of the charging on the part of the electric vehicle 14 because the maximum voltage of the traction battery 20 has been reached (“battery full”), or a communication that charging needs to be terminated, for example owing to severe cold, or because a component in the electrical vehicle 14 is at risk of being destroyed.
For the case where the radio link is interrupted, for example owing to an externally acting fault, or where a communication communicated by one of the two radio subscribers is interpreted incorrectly by the other radio subscriber, or where the vehicle-side or the charging device-side radio device itself has a fault, there is still no completely safe method for dealing with the fault. A termination request transmitted, for example, from the electric vehicle to the stationary charging system device would not be correctly received by said stationary charging system device or would not be received at all thereby or would be interpreted incorrectly thereby. In this case, there is the risk on the vehicle side of a system part of the battery charging system (see FIG. 1) being irreversibly destroyed or even of the possibility of more hazardous states such as a fire or an explosion, for example, occurring.
In order to reduce these risks, it is conceivable to provide a second, redundant transmission path. However, this possible solution results in additional complexity and costs and nevertheless does not provide complete safety, in particular for the vehicle-side system parts.
A known approach for reducing the risks as regards the operation of the radio link involves the vehicle-side or the charging station-side radio device exchanging so-called live signals at regular time intervals, in a manner comparable to a so-called watchdog method, so that the operation and/or stability of the radio link can be checked regularly. In the event of an absence of a live signal expected at a specific time interval, the system expecting the signal can be transferred to a safe state, for example the power transmission into the wireless link can be shut down on the side of the stationary charging system device 220 or the passing-on of the received power via the current rectifier 96 into the traction battery 20 (see FIG. 2) can be shut down on the side of the electric vehicle 14.
A further known approach for reducing risks is based on the consideration that the two radio subscribers are coupled to one another via the wireless link and one radio subscriber has at least approximate knowledge of the electrical state, including an output or drawn electric power, for example, of the respective other radio subscriber. If the present state changes drastically suddenly owing to a problem or a fault, one radio subscriber can be transferred to a safe state or “blocking state”, including, for example, primary power limitation, even without an existing communication link.
Further known approaches for reducing risks or for bringing about a safe state firstly include the connection of discharge resistors in the longitudinal direction of the current retransmission for relieving the current loading on respective downstream components, in the direction of the current flow, i.e. the longitudinal direction of the current retransmission from the first LC resonant circuit 28 acting as receiver via the current rectifier 96 into the traction battery 20, and secondly in the connection of actively switching interrupters, for example contactors, in the longitudinal direction of the current retransmission. Such additionally switched interrupters, in particular contactors, have the disadvantage that they are relatively large, heavy and expensive and that overvoltage peaks may occur in the first LC resonant circuit 28 during implementation of a switching operation, i.e. interruption of the current retransmission in the longitudinal direction thereof.